The present invention relates to an optical pickup provided in information recording/playback apparatus using optical recording media such as a DVD, MD and CD and a high-frequency superposition module.
A laser diode used as a laser light source of an optical pickup is driven in the single mode when it is driven by a direct current. The single mode basically means that a single optical output of a particular wavelength is obtained. However, when the laser diode is driven in the single mode, a rise in the temperature of the laser diode gradually shifts the center of oscillating wavelength toward the longer wavelength at certain wavelength intervals. The variation in the oscillating wavelength is called the mode popping and a noise generated by the mode popping is called the popping noise. In the case of a laser diode used for DVDs, the oscillating wavelength is 650 nm and the mode interval is 0.1 nm. The initial wavelength of 650 at activation varies for example to 650.1 nm due to a rise in temperature. Mode popping often occurs when the temperature of the diode varies. Thus mode popping may be regarded as a laser noise associated with variation in temperature.
Optical output from a laser diode is the sum of the outputs of oscillating wave length modes as longitudinal modes. Optical output is more likely to vary so that a noise is more likely to occur due to variations in temperature in the single mode where a single oscillating wavelength is used than in the multimode where a number of oscillating wavelength are used.
For such a reason, for example, the Japanese patent Publication JP-B-59-9086/(1984) discloses superposition a high-frequency superposition module current on a direct current that drives a laser diode to drive the laser diode in the multi mode. FIG. 6A shows the block diagram of a related art circuit configuration of a high-frequency superposition module that drives a laser diode with this high frequency superposed. FIG. 6B is a circuit diagram of FIG. 6A.
In FIGS. 6A and 6B, a numeral 1 represents a laser diode, 2 a photo-detector diode, and 3 a high-frequency superposition module for driving these diodes. The high-frequency superposition module 3 has a power supply terminal 4 for feeding power to drive the photo-detector diode 2, a power supply terminal 5 for feeding a direct current to the laser diode 1 via a filter 6, an oscillating circuit 7 for feeding a high-frequency current to the laser diode 1, a direct current power supply terminal 8 of the oscillating circuit 7, a filter 9 for preventing the oscillating high frequency from being fed back to the power supply terminal 8, and an impedance matching circuit 10 for providing impedance matching with the circuit on the side on the laser diode 1 and preventing reflection.
As shown in FIG. 6B, the filters 6, 9 are composed of inductors L3, L4, respectively. The oscillating circuit 7 is composed of a transistor Q1, capacitors C1 through C3 and C6, inductors L1, L2, and resistors R1 through R3. The impedance matching circuit 10 is composed of capacitors C4, C5.
The oscillating circuit 7 composes an oscillating circuit that uses an LC resonance phenomenon. The oscillating circuit 7 oscillates at a frequency of several hundreds megahertz and superposes a high frequency on a direct current from the power supply terminal 5 thus driving the laser diode 1. Optical output generated by the laser diode 1 has peaks in a plurality of wavelengths of predetermined wavelength intervals. The envelope of the peaks forms the shape of a crest. That is, multiple longitudinal modes for a laser beam are used to prevent the mode popping noise.
In the circuit of the FIG. 6B, a power supply connected to the power supply terminal 5 for feeding a direct current to the laser diode 1 is separate from the power supply of the oscillating circuit 7 connected to the power supply terminal 8. Thus, turning off the laser diode 1 does not halt continuous operation of the oscillating circuit 7. To halt the operation, it is necessary to provide a power switch of the oscillating circuit 7 apart from the power switch for feeding a direct current to the laser diode 1, and to turn on/off theses switches at the same time.
As mentioned above, a related art high-frequency superposition module comprises a power supply terminal 5 for driving the laser diode by using a direct current and a power supply terminal 8 for driving the oscillating circuit 7. Further, it is necessary to provide filters 6, 9 for preventing propagation of a high frequency into each power supply terminal 5, 8. This makes it difficult to scale down the high-frequency superposition module.
When only the power supply for feeding a direct current to the laser diode is turned on or off, the oscillating circuit remains operating thus causing a noise. To prevent this noise, it is necessary to provide a switch for the oscillating circuit 7 separate from the power supply for feeding a direct current to the laser diode. This complicates the configuration of the optical pickup.